The First Rule of human food consumtion. If you drop the twinkie on the floor of your apartment and pick it up with 3 seconds, it's still good. If you try to put it in your mouth at any point past 3:01 seconds, you're a dirty bastard, and will probably die from gangrene of the mouth.

NOTE: This rule is not applicable in a gas station restroom (that's just wrong).

The Urban Dictionary T-Shirt

n. A rule in which applies that after you spot a woman you would like to meet or notice a woman is giving you eye contact, you immediately approach her within 3 seconds to prevent any bad thoughts or limiting beliefs (excuses) from holding you back.

I was at a club and I noticed that girl checking me out, I gave her the 3 second rule ..1..2..3, then asked her for a female opinion on something.

The Urban Dictionary T-Shirt

The 3 second rule is employed by anal retentives who are not as adventurous or courageous OR hungry enough to utilize the more spunky 5- and 10-second rule. If you are so cautious as to abide by the 3-second rule, maybe you are not cool enough to eat off the floor in the first place.

Adam: I am so hungry because I dropped my meatpie on the floor and it has been 4 seconds and I live by the 3 second rule.

Rebecca and Cristina: No problem. We will eat it-- we even have time to tie our shoelaces first while we are down there. Mmmmmm delicious.

The Urban Dictionary T-Shirt

The nationally acknowledged time in which after leaving a seat unoccupied it is then acceptable for people to take your seat, usually in a pub or somewhere which has a variety of good chairs and bad chairs. Any attempt to do so before this is an illegal move and that person will have to leave the seat. Although this rule is acceptable amongst your group you are sat with, any attempt to utilise this rule with strangers is not seen as a good idea.

Ahmed "What you doin lad? That is my seat."
Swann "3 second rule. You were at the bar for 5 minutes."
Ahmed "Ah fair play mate. I'll just sit here instead then."